The invention relates to a method of correcting measurements from current transformers at inputs and outputs of a current node defined by an area and by one phase of a set of busbars. The method is intended to limit the risk of unnecessary tripping of a differential protection system assigned to one phase current of the set of busbars.
Conventional differential protection systems calculate, during regular tests, the vector sum of the currents of a phase to determine the differential current Id between the inputs and outputs of the protected area. A null differential current is sensed in the absence of faults in the area and a significant differential current is sensed in the event of a short circuit generating a fault current or in the event of a leakage current to earth on a busbar carrying the phase. The detection of real fault currents (including leakage currents) is relatively reliable with a differential protection system assigned to an area comprising a limited number of feeder bays, and therefore a limited number of current transformers, typically ten transformers. On the other hand, if the set of busbars includes a large number NTC of current transformers in the protected area, a conventional differential protection system is not protected from unnecessary tripping resulting from the absence of statistical compensation for the gain errors of each transformer. Most feeder bays of a substation are two-way and can therefore supply or consume energy. For a given phase, the sum of the sensed currents entering an area is equal to the sum of the sensed currents leaving it. Current transformers usually have a positive or negative gain error dg and the measured current (I)0 is therefore equal to Ixc3x97(1xc2x1dg), where I is the true current. Assuming an extreme case in which all the input transformers have a mean positive gain error equal to dg0 and all the output transformers have a mean negative gain error equal to xe2x88x92dg0, and assuming for simplicity that there are the same number of input and output transformers and that each transformer is carrying the same current I, an erroneous differential current (Id)0 equal to 1xc3x97NTCxc3x97dg0 is measured, which is not acceptable if the set of busbars includes a large number NTC of transformers. This is because, if dg0 is of the order of 1% and NTC is equal to about fifty, the erroneous differential current (Id)0 measured is approximately equal to half the current I passing through a transformer. The differential protection system cannot distinguish this erroneous current from the real differential current caused by a busbar fault, and is therefore tripped even in the absence of a fault.
To remedy the above drawbacks, the applicant has developed a method of statistically balancing current transformer measurements in order to apply statistical correction to the errors of transformers assigned to differential busbar protection. The statistical nature of the correction implies that, although errors are corrected globally, an individual correction to the initial measurement from a transformer can lead to a value that is worse than the initial value, i.e. farther from the true value. Nevertheless, the set of corrections statistically balances the global error for all the transformers, which yields a corrected measurement of the differential current for which the residual error is only a small percentage, typically of the order of the percentage gain error for a single transformer.
The invention therefore provides a method of correcting measurements from current transformers disposed at inputs and outputs of a current node defined by an area and by one phase of a set of busbars, intended to limit the risk of unnecessary tripping of a differential phase current protection system, said differential protection system calculating the vector sum of the currents to determine the differential current Id between the inputs and outputs of said area in the course of regular tests, which method is based on a statistical balancing method using measured current vectors (Inxe2x86x92)0 each obtained from a measurement of the current In and the relative phase fn of the real current vector Inxe2x86x92 supplied at the time of each test by a current transformer CTn, and it uses at the time of at least one test an iterative convergence algorithm for determining from the erroneous differential current vector (Idxe2x86x92)0 formed by the sum xcexa3(Inxe2x86x92)0 of the measured current vectors the correction vector xcex5Inxe2x86x92 to be applied to each measured current (Inxe2x86x92)0 to obtain a corrected current vector Inxe2x86x92c substantially equal to the real current vector Inxe2x86x92. The statistical balancing method corrects said erroneous differential current vector (Idxe2x86x92)0 statistically by adding to it the sum of the correction vectors xcex5Inxe2x86x92.
In a preferred mode of application of the method according to the invention, on each iteration k and for each iterative current vector (Inxe2x86x92)k calculated that converges towards the real current vector Inxe2x86x92 from iteration zero using the measured current vectors (Inxe2x86x92)0, the iterative convergence algorithm determines a partial correction vector (xcex5Inxe2x86x92)k to be added to said iterative current vector (Inxe2x86x92)k to form the current vector (Inxe2x86x92)k+1 of the next iteration k+1, said partial correction vector (xcex5Inxe2x86x92)k being collinear with the erroneous differential current vector (Idxe2x86x92)0 of the opposite direction whose normalized value is the same as that of the normalized vector product of each iterative current vector (Inxe2x86x92)k and the iterative erroneous current vector (Idxe2x86x92)k weighted by a coefficient xcex affecting the rate of convergence of the algorithm and in the range from zero to one.
In another mode of application of the method according to the invention, a new test of the set of currents of the protected area is carried out during a series of tests as soon as two correction vectors (xcex5Inxe2x86x92)t1 and (xcex5Inxe2x86x92)t2 calculated over two consecutive tests of the real current vector Inxe2x86x92 of the same current transformer CTn correspond to two gain corrections (dgn)t1 and (dgn)t2 whose percentage difference is greater than a particular reference value r, and the statistical balancing method is applied for a statistically greater number NT of tests as the number Nc of currents in the protected area increases.
In another mode of application of the method according to the invention, a partial correction vector (xcex5Inxe2x86x92)k obtained at the time of an iteration is ignored if its modulus is greater than a particular percentage of the modulus of the erroneous differential current vector (Idxe2x86x92)0, in order to be able to distinguish a differential current caused by a fault on the set of busbars from the erroneous differential current (Idxe2x86x92)0.
In another mode of application of the method according to the invention, the correction vectors xcex5Inxe2x86x92 obtained for the set of current vectors (Inxe2x86x92)0 measured at the time of a test are processed to provide for each current transformer CTn an instantaneous gain correction matrix and an instantaneous relative phase correction matrix to be applied respectively to the gain and the phase-shift introduced by the transformer. A stabilized gain or relative phase correction matrix is produced for each current transformer CTn at the end of a series of tests and corresponds to the convergence of the instantaneous correction matrices of the current transformer obtained during the tests of the series.
In another mode of application of the method according to the invention, tabulated data and functions of the various configurations of the available numbers NC of currents and NT of tests provide an indication of the quality of convergence in modulus and in phase of the statistical correction to be applied to the measured differential current vector, said statistical correction being calculated after a particular number NT of tests and converging towards the optimum statistical correction, which is obtained after a number NT of tests that increases as the number NC of currents increases.